1. Field Of The Invention
This invention generally relates to signal controllers, and more specifically relates to a vector modulator which is suitable for use as an RF signal controller in an interference cancellation system. Even more specifically, this invention relates to a vector modulator particularly adapted for high speed, wide dynamic range applications.
2. Description of The Prior Art
A vector modulator is sometimes referred to as a signal controller. It is a circuit which primarily functions to adjust the amplitude and phase of a signal.
One of the primary applications of a vector modulator is in an interference cancellation system, such as disclosed in U.S. Pat. No. 3,699,444, which issued to Rabindra Ghose et al. In such an interference cancellation system, the vector modulator generally receives a sample of a transmitter signal, which may be causing interference in a collocated receiver system, and adjusts the amplitude and phase of the sampled signal to provide a cancellation or correction signal which is essentially the negative complement of the transmitter signal. The cancellation signal is injected into the signal path of the receiver system to cancel or at least minimize the interference caused by the transmitter signal.
Generally, the vector modulator works in conjunction with a synchronous or coherent detector and a loop amplifier and filter. The detector compares a sample of the received signal with that of the transmitter signal and provides control signals to the vector modulator via the control loops that minimize the synchronous detection products of the two sampled signals. The vector modulator adjusts the amplitude and phase of the sampled transmitter signal in accordance with the control signals it receives from the control loops.
More specifically, the vector modulator adjusts the amplitude of the in-phase and quadrature phase, or sine and cosine, components of the reference or sampled transmitter signal. By so doing, a cancellation signal may be provided with the desired amplitude and phase angle. Accordingly, a typical vector modulator, such as disclosed in the Ghose et al. patent, includes two signal controllers 39, 40, each for respectively modifying the in-phase or sine component and the quadrature phase or cosine component of the sampled signal.
An early form of a signal controller is disclosed in U.S. Pat. No. 3,550,041, which issued to Walter Sauter. FIG. 1 of this patent shows a signal controller having a transformer 13 which basically provides two output signals corresponding to the input RF signal, which output signals have a phase angle difference of 180.degree.. The secondary winding of the transformer 13 is connected to two PIN diodes 20, 21. A DC bias signal provided to each PIN diode allows one or the other of the output signals to be selected. By adjusting the bias signals, attenuation of the selected signal may be controlled. In effect, two such signal controllers would be required in forming a vector modulator, each controller being employed to adjust in amplitude either the in-phase or the quadrature phase component of the RF signal.
U.S. Pat. No. 4,016,516, which issued to Walter Sauter, et al., discloses another conventional vector modulator, which is referred to in the patent as a signal controller. As shown in FIG. 3 of the Sauter, et al. patent, a quadrature hybrid 51 is used to produce the in-phase and quadrature phase components of a reference signal, and an in-phase hybrid or zero degree summer 60 is used to sum the components back together.
Each of the in-phase and quadrature phase components is adjusted in amplitude by using a quadrature hybrid 55, 56 and a pair of PIN diodes 61, 62 and 63, 64, used as terminations. The bias of the PIN diodes is controlled in a manner similar to that described in U.S. Pat. No. 3,550,041 to adjust the amount of the in-phase and quadrature phase components which is absorbed by the PIN diodes and the amount which is reflected through the quadrature hybrids 55, 56 and summed in the zero degree summer 60 to produce a cancellation signal.
The signal controllers (or vector modulators) disclosed in U.S. Pat. Nos. 3,550,041 and 4,016,516 work well in many applications. However, the application of interference cancellation techniques to frequency agile and broad instantaneous bandwidth systems requires increased vector modulator performance, and in such applications, the signal controllers disclosed in the aforementioned patents may have limited capabilities due primarily to the PIN diodes employed in the controllers, and their associated insertion phase variation over the amplitude control range.
Many conventional interference cancellation systems are of the fixed geometry, fixed frequency type. That is, the geometry between the interfering signal source and the receiver antenna is fixed, and also such interference cancellation systems tend to operate with a very narrow bandwidth and at a particular frequency. Once the system is tuned up, there is relatively little change in the in-phase and quadrature phase loops of the system that the vector modulator would experience.
However, in an interference cancellation system which must be able to respond to many different frequencies and in which the antennas have a different response at such frequencies, the in-phase and quadrature phase control values may change radically. The vector modulator must be capable of responding quickly and accurately to such changes.
Thus, frequency agile systems require the vector modulator to respond quickly after being commanded from a previous setting. PIN diodes have relatively large parasitic capacitances, which tend to slow down the response time of the signal controller or vector modulator employing the diodes, as it is difficult to move charge quickly in and out of the diode's capacitance. Smaller geometry PIN diodes may be employed in the vector modulator to improve response time, but not without a reduction in power handling capability and a corresponding increase in the amplitude of distortion products.
In many interference cancellation system applications, vector modulators must be capable of handling large input signal powers. To handle such large input powers, conventional vector modulators would require large geometry PIN diodes. Larger geometry PIN diodes correspondingly have greater parasitic capacitance, which tends to slow down the response time of the vector modulator. Accordingly, there is a speed/power tradeoff when using a PIN diode in a vector modulator.
Another problem with PIN diode type vector modulators is that the phase and amplitude response of the vector modulator may vary with respect to frequency. PIN diodes, especially those which are capable of handling larger power levels, are fairly large devices. Many PIN diodes tend to be discrete components, as opposed to monolithically formed chip type devices, and thus have lead inductances and parasitic capacitances associated with them. Their intrinsic characteristics form resonant circuits, which may cause the vector modulator to change in its response as a function of the frequency of the signal it is handling. This affects the performance characteristics of the interference cancellation system in which the vector modulator is used.
Similarly, the phase and amplitude response of conventional PIN diode modulators may be affected by thermal drift in the PIN diodes. In conventional vector modulators, the PIN diodes must absorb a sizeable portion of the signal power. The diodes tend to heat up, which may cause their electrical characteristics, such as their RF resistance, to change. As a result, the phase and amplitude correction the PIN diode modulator imparts to the RF signal may differ from the correction expected from the values applied to the bias or control ports of the vector modulator by the control loops.
Another problem with many conventional vector modulators used in interference cancellation systems is that their insertion phase tends to vary as a function of the attenuation they apply to the signal. If the insertion phase of each of the in-phase and quadrature phase paths through the vector modulator is not independent of its attenuation setting, the relationship between the in-phase and quadrature phase components will deviate from 90.degree. after passing through the signal controller portions of the modulator. Furthermore, each component will differ in phase from its counterpart in the synchronous detector. Accordingly, a change in the in-phase component due to the insertion phase shift will affect the quadrature phase component. The quadrature phase signal control loop of the interference cancellation system will accordingly make a correction to the quadrature phase component, and that correction will, in turn, impact the in-phase signal control loop of the system. In other words, the two system control loops will no longer be independent of each other. A change in the output of one control loop will affect the other, and this will prolong the "settling time" of the entire cancellation system.